On CD (Compact Disc) or DVD (Digital Versatile Disc), or other disk type optical recording medium (hereinafter referred to as optical disc), multiple narrow bumps known as bits having a length corresponding to the recorded data are formed on the recording surface of the disc irradiated with a laser beam. The bits are configured in a spiral composed of columns arranged on the recording surface from the center to the periphery corresponding to the recording order of data. When the recorded data are read from the optical disc, a laser beam is irradiated on the bit columns. The intensity of the irradiated laser beam is lower when the laser beam spot is on a bit than when it is on the flat surface area outside the bits. The intensity of the reflected light is converted into an electronic signal by means of a photodiode or other optical detector. In this way, the information recorded on the optical disc is electrically regenerated. The electronic signal output corresponding to the intensity of reflected light from the optical detector is modulated at a high frequency in accordance with whether or not the laser spot is on a bit area. Consequently, it is also called “RF signal.”
Each completed revolution of the bit columns arranged side by side in a spiral configuration is called a track. In the regenerating apparatus of an optical disc, such as a CD, DVD, etc., there is a function that allows regeneration of the recorded data by skipping said tracks. One of characteristic feature of the disc type optical recording medium is that regeneration can be performed at higher speed than a tape type recording medium.
In order to jump over tracks so as to regenerate the information recorded on the desired position of the disc, it is necessary to know the present track where the laser beam spot is located and the target track to be reached by the laser beam spot after jumping over a prescribed number of tracks. In a conventional optical disc regenerating apparatus, there is a circuit known as a mirror detection circuit that detects whether the light spot is on a track or on the flat mirror portion outside the tracks. As the laser beam spot passes over the tracks, the number of track portions and mirror portions is counted by the mirror detection circuit, which detects these alternating portions. In this way, the number of the tracks that have been traversed is known.
FIG. 5 is a schematic block diagram illustrating an example of the constitution of a conventional mirror detecting circuit and defect detecting circuit.
Mirror detecting circuit (200a) shown in FIG. 5 has peak-hold circuit (102), peak-hold circuit (104), voltage divider (106), offset circuit (107), and comparator (109). Also, defect detecting circuit (200b) shown in FIG. 5 has peak-hold circuit (101), voltage divider (105), and comparator (108). Peak-hold circuit (102) and peak-hold circuit (103) are shared with mirror detecting circuit (200a).
Peak-hold circuits (101)–(104) are circuits that hold the maximum peak level (hereinafter referred to as top level) or the minimum peak level (hereinafter referred to as bottom level) of RF signal Srf, the electronic signal obtained by conversion using an optical detector. Peak-hold circuits (101) and (102) hold the top level of RF signal Srf, while peak-hold circuits (103) and (104) hold the bottom level of RF signal Srf.
Also, the droop rate that indicates the rate of attenuation of the hold level of the peak-hold circuit with lapse of time is set individually for each peak-hold circuit. Properties of the signals held in the various peak-hold circuits are different from each other corresponding to values of the droop rates.
The droop rate of peak-hold circuit (101) is set relatively low for output of top-hold signal Sth that keeps a constant top level for RF signal Srf. For example, the droop rate may be set at about 1 msec/V.
The droop rate of peak-hold circuit (102) is set higher than that of peak-hold circuit (101) so that top envelope signal Ste is output corresponding to the envelope that describes the top level of RF signal Srf. For example, the droop rate may be set at about 100 μsec/V.
The droop rate of peak-hold circuit (103) is set relatively low for output of bottom-hold signal Sbh that keeps a constant bottom level for RF signal Srf. For example, the droop rate may be set at about 10 msec/V.
The droop rate of peak-hold circuit (104) is set higher than the rates of peak-hold circuits (102) and (103) so that bottom envelope signal Sbe is output corresponding to the envelope that describes the bottom level of RF signal Srf. For example, the droop rate may be set at about 10 μsec/V.
Voltage divider (105) voltage-divides at a prescribed ratio for top-hold signal Sth of peak-hold circuit (101) and bottom-hold signal Sbh of peak-hold circuit (103), and the obtained defect detection threshold signal Sdt is input to the positive terminal of comparator (108).
Voltage divider (106) divides voltage at a prescribed ratio for top envelope signal Ste of peak-hold circuit (102) and bottom-hold signal Sbh of peak-hold circuit (103), and the obtained signal is input to offset circuit (107).
Offset circuit (107) adds a prescribed offset to the voltage division signal output from voltage divider (106), and the obtained signal is input as mirror detection threshold signal Smt to the negative terminal of comparator (109).
Comparators (108) and (109) compare the magnitudes of the signal levels input to the positive terminal and the negative terminal, and output a signal with a logic value of “1” when the signal level at the positive terminal is higher than that at the negative terminal, and output a signal with a logic value of “0” when the signal level at the positive terminal is lower than that at the negative terminal.
When top envelope signal Ste is smaller than said defect detection threshold signal Smd, comparator (108) outputs defect detection signal Sd with a logic value of “1” to indicate detection of said defect on the optical disc.
When bottom envelope signal Sbe is larger than said mirror detection threshold signal Smt, comparator (109) outputs mirror detection signal Sm with a logic value of “1” to indicate detection of a mirror portion.
In the following, operation of mirror detecting circuit (200a) and defect detecting circuit (200b) having the aforementioned constitution will be explained.
FIG. 6 is a waveform diagram illustrating the operation of mirror detecting circuit (200a) shown in FIG. 5. In the figure, the ordinate represents the signal level, and the abscissa represents time.
FIG. 6a illustrates a waveform example of RF signal Srf. FIG. 6b shows top envelope signal Ste, bottom-hold signal Sbh, and bottom envelope signal Sbe obtained by peak holding of RF signal Srf shown in FIG. 6a by peak-hold circuits (102)–(104), as well as said mirror detection threshold signal Smt. FIG. 6c illustrates a waveform example of mirror detection signal Sm output from comparator (109).
As shown in FIG. 6a, the signal amplitude of RF signal Srf output from the optical detector will be different when the light spot is on a track and when the light spot is on the mirror portion between tracks.
When the light spot is on a track, as shown at peak position P4 in FIG. 6a, the amplitude of RF signal Srf is higher. This is due to the large difference in the intensity of the reflected light comparing the case when the light spot is on a bit to the case when the light spot is outside the bit region. Also, because the intensity of reflected light corresponds to the lowest level of RF signal Srf when the light spot is directly on a bit, in this case, the bottom level of RF signal Srf is lowest.
On the other hand, when the light spot is on the mirror portion between tracks, as shown at peak position P3 in FIG. 6a, the amplitude of RF signal Srf is smaller, and at the same time, the bottom level is higher. In a conventional optical disc regenerating apparatus, even when the light spot is at the center of the mirror portion, it still partially overlaps the adjacent track, and the intensity of the reflected light in the overlapped portion is modulated, so that there is certain high-frequency modulation component of RF signal Srf even at peak position P3 of the mirror portion, as shown in FIG. 6a. 
As shown in FIG. 6b, top envelope signal Ste has a waveform corresponding to the envelope of the top level of RF signal Srf, and bottom envelope signal Sbe has a waveform corresponding to the envelope of the bottom level of RF signal Srf. Also, bottom-hold signal Sbh has a waveform that holds the bottom level of RF signal Srf.
Mirror detection threshold signal Smt is a signal obtained by voltage dividing top envelope signal Ste and bottom-hold signal Sbh using voltage divider (106), and it gives a prescribed offset in offset circuit (107). It has a signal level of a prescribed proportion between top envelope signal Ste and bottom-hold signal Sbh. In the example shown in FIG. 6b, the signal level is between the crest and trough of bottom envelope signal Sbe.
Mirror detection signal Sm is output from signal comparator (109) when bottom envelope signal Sbe is compared with mirror detection threshold signal Smt. In the example shown in FIG. 6c, the output is high (with a logic value of “1”) when the signal level of bottom envelope signal Sbe is above mirror detection threshold signal Smt, and low (with a logic value of “0”) when it is below the threshold.
When the light spot passes over the region on an optical disc with intensity of reflected light reduced due to the presence of fingerprints, smudges, etc., as shown in period T1 in FIG. 6a, the overall amplitude of RF signal Srf is reduced. In this case, since the amplitudes of top envelope signal Ste and bottom envelope signal Sbe decrease in the same relative proportion, the signal level of mirror detection threshold signal Smt also decreases in the same proportion. That is, even when the amplitude of bottom envelope signal Sbe decreases due to the decrease in the intensity of reflected light caused by smudges, the level of mirror detection threshold signal Smt decreases in the same proportion as that of said decrease in the amplitude, mirror detection can still be performed. Consequently, it is possible to suppress the decrease in the mirror detection sensitivity with respect to smudges on the disc surface.
FIG. 7 is a waveform diagram illustrating the operation of defect detecting circuit (200b) shown in FIG. 5. In this figure, the ordinate represents the signal level, and the abscissa represents time.
FIG. 7a is a diagram illustrating a waveform example of RF signal Srf. FIG. 7b illustrates waveform example of peak holding signal Sth, top envelope signal Ste and bottom-hold signal Sbh obtained by peak-holding RF signal Srf of FIG. 7a using peak-hold circuits (101)–(103). FIG. 7c illustrates waveform example of defect detection signal Sd output from comparator (108).
In period T2 shown in FIG. 7a, since the light spot passes over the region on the optical disc where the intensity of reflected light decreases significantly due to scratches or adherence of dust, etc., the overall amplitude of RF signal Srf decreases. As shown in FIG. 7b, top-hold signal Sth and bottom-hold signal Sbh are kept to almost a constant signal level during period T2 because the droop rate is low for peak-hold circuits (101) and (103). Consequently, the signal level of defect detection threshold signal Sdt obtained by voltage dividing at a prescribed proportion using voltage divider (105) is kept constant during period T2. When the intensity of the reflected light falls, the signal level of top envelope signal Ste decreases. However, when said constant defect detection threshold signal Sdt decreases, as shown in FIG. 7c, defect detection signal Sd goes high (with a logic value of “1”), and the defect on the optical disc is detected.
The track pitch of DVD is 0.74 μm, less than half of 1.6 μm of CD. On the other hand, the wavelength of the laser beam used for regeneration of DVD is 650 nm, about 20% shorter than the wavelength of 780 nm for CD. Consequently, the ratio of light spot diameter to the track pitch of DVD is larger than that of CD. Also, since the pitch width of DVD is 0.3 μm, larger than half of the pitch width of 0.5 μm of CD, the ratio of the width of the mirror portion to the track pitch of DVD is smaller than that of CD. That is, even when the light spot is at the center of the mirror portion of a DVD, the intensity of the reflected light modulated by the pitch of the adjacent track is increased more than that in a CD. Consequently, the amplitude of RF signal Srf is larger, and the difference in the bottom level between the mirror portion and the track portion is smaller. That is, the amplitude of bottom envelope signal Sbe decreases.
FIG. 8a illustrates waveform example of RF signal Srf of CD. FIG. 8b illustrates waveform example of RF signal Srf of DVD. As can be seen from these figures, the difference in the bottom level between the case when the light spot is at the center of the track (on-track) and the case when it is at the center of the mirror portion (off-track) of DVD is less than that in a CD. Consequently, in comparator (109), a sufficient level difference for performing the comparison operation may not be obtained. For example, when the overall amplitude of RF signal Srf decreases as shown in period T1 of FIG. 6 due to smudges, etc., on the disc surface, the sensitivity of mirror detection on DVD deteriorates significantly, and it may be impossible to perform normal track jumping for regeneration. That is, in mirror detecting circuit (200a) shown in FIG. 5, the track pitch of DVD, etc., becomes narrower, and the mirror detection sensitivity of the optical disc deteriorates. Consequently, due to noise and smudges on the disc, the mirror portion may be difficult to detect, which is undesirable.
In the following, other conventional mirror detecting circuits for solving the aforementioned problems of mirror detecting circuit (200a) shown in FIG. 5 will be explained.
FIG. 9 is a block diagram illustrating schematically another structural example of the conventional mirror detecting circuit.
As shown in FIG. 9, mirror detecting circuit (300) has low-pass filter (109), gain control amplifier (110), gain control amplifier (112), capacitor (111), peak-hold circuit (113), peak-hold circuit (114), voltage divider (115), offset circuit (116) and comparator (117).
Low-pass filter (109) removes the modulation component in the high-frequency region of input RF signal Srf, and extracts the signal component that varies corresponding to the track and mirror portion and outputs it to gain control amplifier (110) when the light spot passes over the tracks.
Gain control amplifier (110) amplifies signal S10 output from low-pass filter (109) with a prescribed gain, and outputs it to capacitor (111). The gain of gain control amplifier (110) is set corresponding to the type of optical disc evaluated by an optical disc type evaluation circuit (not shown in the figure).
Capacitor (111) removes DC component from the signal output by gain control amplifier (110), and outputs the AC component to gain control amplifier (112).
Gain control amplifier (112) amplifies the output signal of gain control amplifier (110) without the DC component, with a prescribed gain, and outputs the amplified signal to comparator (117), peak-hold circuit (113) and peak-hold circuit (114). The gain of gain control amplifier (112) is determined in accordance with the type of optical disc evaluated by said optical disc type evaluation circuit.
Peak-hold circuit (113) holds the top level of signal S11 output from gain control amplifier (112) at a prescribed droop rate, and outputs it to voltage divider (115).
Peak-hold circuit (114) holds the bottom level of signal S11 output from gain control amplifier (112) at a prescribed droop rate, and outputs it to voltage divider (115).
Voltage divider (115) voltage-divides, in a prescribed ratio, signal S12 of the top level of signal S11 held by peak-hold circuit (113) and signal S13 of the bottom level of signal S11 held by peak-hold circuit (114), and outputs the voltage-divided signal to offset circuit (116).
Offset circuit (116) outputs mirror detection through signal S14 that gives the prescribed offset to the voltage divided signal from voltage divider (115) to comparator (117).
Comparator (117) compares output signal S11 of gain control amplifier (112) and mirror detection threshold signal S14 from the offset circuit, and outputs high-level mirror detection signal Sm when output signal S11 is above mirror detection threshold signal S14, and outputs low-level mirror detection signal Sm when output signal is below mirror detection threshold signal.
The operation of mirror detecting circuit (300) shown in FIG. 9 with the aforementioned constitution will be explained below.
FIG. 10 is a waveform diagram illustrating the operation of mirror detecting circuit (300) shown in FIG. 9. In this figure, the ordinate represents the signal level, and the abscissa represents time.
FIG. 10a illustrates waveform example of RF signal Srf input to low-pass filter (109). FIG. 10b illustrates the waveform of signal S10, with the high-frequency modulation component contained in said RF signal Srf removed by low-pass filter (109). After signal S10 is amplified by gain control amplifier (110), the DC component is removed by capacitor (111), and the resulting signal is input to gain control amplifier (112).
FIG. 10c illustrates the waveforms of signal S12 that holds the top level of signal S11 output from gain control amplifier (112), signal S13 that holds the bottom level, and mirror detection threshold signal S14 that is obtained by voltage-dividing said top level signal S12 and bottom level signal S13 using said voltage divider. As shown in FIG. 10d, mirror detection signal Sm goes high when the level of signal S11 is above the level of mirror detection threshold signal S14, and it goes low when the level of signal is below the level of mirror detection threshold signal.
In mirror detecting circuit (300) shown in FIG. 9, variations in amplitude corresponding to the track and mirror portions contained in RF signal Srf can be amplified to an appropriate level by gain control amplifiers (110) and (112), and the level difference in the signal input to the comparator in the last stage can be increased. In this way, it is possible to increase the mirror detection sensitivity as compared with mirror detecting circuit (200a) shown in FIG. 5.
However, mirror detecting circuit (300) of FIG. 9 has the following problems.
FIG. 11 is a waveform diagram illustrating the problems of mirror detecting circuit (300) shown in FIG. 6.
FIG. 11a is a diagram illustrating a waveform example of input RF signal Srf. In this example, a vibrational component other than the amplitude component corresponding to the track and mirror portions at the bottom level appears at the top level of RF signal Srf. As shown in this waveform example, when the top level vibrates with a frequency component near the vibrational component at the bottom level, signal S10 output from low-pass filter (109) becomes a signal which has the signal component at the top level, which is undesirable for mirror detection, overlapped on the vibrational component of the bottom level. In this way, detection of the mirror portion becomes difficult, which is undesirable.
FIG. 11b illustrates waveforms of output signal S11 of gain control amplifier (112), top-hold signal S12, bottom-hold signal S13, and mirror detection threshold signal S14 in the case of single-track jump operation for only one track. As shown in FIG. 11b, when the single-track jump operation is performed, at the initial stage, the signal level held by peak-hold circuits (113) and (114) is still at the bottom level. Consequently, at time t1 before rise of mirror detection threshold signal S14 to the steady state, the level of signal S11 exceeds mirror detection threshold signal S14, and mirror detection signal Sm rises to the high level, as shown in FIG. 11c. 
On the other hand, when a jump operation is performed over multiple tracks, at time t2 when mirror detection threshold signal S14, that has risen to the steady state, exceeds signal S11, as shown in FIG. 11d, mirror detection signal Sm′ rises. Consequently, compared with the case of a multiple-track jump operation, in a single-track jump operation, mirror detection signal Sm rises faster, which is undesirable. Mirror detection signal Sm is used not only in counting the number of tracks, but also in controlling the braking movement of the optical pickup on the target track. Consequently, detection error of mirror detection signal Sm influences the braking control, so that it may be impossible for the optical pickup to stop on the target track.
A general object of the present invention is to solve the aforementioned problems of the conventional methods by providing a mirror detection signal generator that can detect the mirror portion with high stability, independently of the type of the optical recording medium.